Preloaded bolts transfer the load through the contact surfaces of the plies. The contact surfaces must be prepared to achieve the satisfactory slip factor which might be determined by test or taken from tables.
According to BS EN 1090-2:2008, the following table might be used to determine the slip factor for different friction surfaces.
This table applies also to packing plates provided to offset differences in thickness.
It should be noted that the surface preparation requirements concern the faying surfaces only, that is the surfaces that resist relative movement of the plies (shown in red in Figure 1). The area under the bolt head and washer are not subject to the same requirements (shown in blue in Figure 1).
Paint is permitted under the bolt heads and washers, but the Research Council on Structural Connections cautions against large coating thicknesses. Tests have also indicated that significant bolt pre-tension may be lost when the total coating thickness approaches 15 mils (381 microns) per surface, and that surface coatings beneath the bolt head and nut can contribute to additional reduction in pre-tension.
Series NG 1900 - Notes for Guidance on the Specification for Highway Works - Protection of Steelwork Against Corrosion states that: “When paint is applied to the outer faces of joint material at preloaded connections, a check should be made that joint plates are not reversed during assembly; also if the maximum dry film thickness of 50 microns of paint is exceeded the whole coat should be taken off before the joint is made, otherwise the load carrying capacity of the joint may not be achieved. This also applies to sealer. “
To be on the safe side, no paint or sealer should be applied under the bolt heads and washers. If an application of paint or sealer is necessary, the total dry film thickness of the coating should not exceed 50 microns.